A magnetic-controlled actuator (100) with an auto-locking function for joints of a manipulation arm mainly includes an inner-layer stator (10), an inner-layer mover (20), an outer-layer mover (30), an outer-layer stator (40), and a fixing shaft (50). The inner-layer mover (20), the outer-layer mover (30), and the outer-layer stator (40) have a plurality of permanent magnets, respectively. The fixed shaft (50) simultaneously penetrates through the inner-layer stator (10), the inner-layer mover (20), the outer-layer mover (30), and the outer-layer stator (40) forming a coaxial arrangement. The inner-layer mover (20) rotates relatively to the inner-layer stator (10) to output power from the actuator (100). Therefore, a cogging effect, which is produced due to interaction of the permanent magnets between the outer-layer mover (30) and the outer-layer stator (40), is automatically produces a high cogging torque for the actuator (100). Thus the actuator (100) provides a sufficient locking force to lock the non-stationary inner-layer mover (20) when a power failure occurs.